Nanotechnology is an increasingly employed concept in the development and progression of a wide variety of technologies, including the field of electrochemistry. Nano-size materials have been investigated and discovered for use as anode materials in energy storage and conversion devices such as electrodes and capacitors. Currently, carbon is a preferred material for use in anodes. Carbon is an inexpensive anode material, however, there are disadvantages associated with the use of carbon. For example, the volumetric power density of carbon is not sufficiently high. Metal, metal alloy and metal oxide nano-composites have been identified as potential alternative anode materials to carbon. It is believed that the discharge capacities of these nano-composites may exceed the known discharge capacities of carbon. Metals can be good electronic conductors and offer the potential to exhibit high gravimetric and volumetric capacitance due to their large molar densities. Metal alloys can be formed at room temperature, for example, if a metal is polarized to a sufficiently negative potential in a Li-ion conducting electrolyte. It is believed that the charge density of some metal alloys may be higher than that of lithiated carbon. However, the potential advantages of these materials have not resulted in their commercial use and carbon remains a preferred anode material. It is believed that the limited progress of metals, metal alloys and metal oxides as alternative anode materials may be due to metals undergoing major changes in structure and volume while alloying. For example, in the formation of Lix+Mx−, the host metal (M) not only accepts several moles of Li per metal but also accommodates negative charges. This process can result in the formation of a brittle alloy. Further, the resultant brittle alloy can undergo a significant volume change (e.g., from 300-600%) between the unalloyed and alloyed states. This change can build mechanical stresses which can result in crumbling and loss of inter-particle electronic contact that may cause capacity loss and fade which may lead to rapid failure of cells.
It is desirable to develop a process for synthesizing nano-particles including metals, such as Si, contained in a nano-structured matrix. It is believed that an active material in an inactive matrix can result in a large capacity as well as the desired reversibility enabling superior performance as compared to carbon as an anode material. Furthermore, it is believed that the presence of carbon nanotubes can contribute to improved performance. The compliant nature of carbon nanotubes and their ability to bend and flex can result in the nanotube maintaining electrical contact with the active material during alloying and de-alloying and thus, preserving the desirable high gravimetric capacity of the active material.
Thus, it is desired to formulate strategies to identify approaches and systems that can demonstrate reversible and stable high capacities while exhibiting other characteristics such as irreversible loss and electrochemical stability.